In a flat display device such as a liquid crystal display device (LCD) and an organic light emitting display device (OLED), a thin film transistor (TFT) acts as a switch device and drive device. The flat display device generally comprises amorphous silicon TFT (a-Si TFT) or polycrystalline silicon TFT (p-Si TFT). a-Si TFT comprises an active layer formed by a-Si, while p-Si TFT comprises an active layer formed by p-Si.
As compared with a-Si, LTPS has significantly higher carrier mobility. This effectively y decreases an area of TFT, improves an aperture ratio. Furthermore, a LTPS display device has a quick response, higher resolution and display quality. LTPS has been widely applied to OLED, especially an active matrix light emitting display device (AMOLED).
Many processes, such as solid state crystallization (SPC), metal induced lateral crystallization (MILC), excimer laser crystallization (ELC) have been applied to fabricate LTPS. In the ELC process, a laser beam (shot) irradiates an amorphous silicon layer, so that amorphous silicon layer is molten in the irradiated region and recrystallized to form a polycrystalline silicon layer.